Difficulty is based on how meany nodes are on the network, correct? Growth will slow when miners realize it is unprofitable to mine, at the same time the value of the Bitcoin will go up thanks to increased scarcity.
It seems to me that the Bitcoin was not designed for stability. Oh well, just means you can make a ton of money real fast... but also lose it.
Difficulty is the nodes multiplied by the average horsepower of the nodes.
In a perfectly rational market what you say would be true. But it already isn't profitable, and people are still joining. I just joined because I bought hardware based upon a bitcoin calculator. Anyway, will be a long time before every gamer that already has a system has joined the network. And for them it is free.
It won't be unprofitable to mine in terms of electricity for another 60 days or so (assuming $20/bitcoin, which could go up or down).
So the gamers will still be piling on for at least that long.
All of this assumes that more and more people will be interested in using bitcoins, which seems to be true so far.